Progress in a light-emitting semiconductor device is remarkable in these days. Especially, a light-emitting diode (LED) has been miniaturized, and has lower power consumption and higher reliability. So an LED is widely used as a display luminous source. The LED which is used practically is made of a III-V group compound semiconductor. Especially a gallium nitride compound semiconductor (AlxGayIn1-x-yN (0≦x≦1, 0≦y≦1, 0≦x+y≦1)) is a direct transition type semiconductor which covers from ultraviolet region to orange region. So it is mainly used as an LED which emits blue or green lights, and large numbers of such LED are employed as a display unit in the open air and a traffic signal. If efficiency of the LED is further promoted, it becomes to have larger possibility to be employed as excitation light source for a fluorescent lamp. Now expectation for a mercury (Hg)-free LED is growing from an environmental view, development of LEDs is proceeded vigorously.
Generally, a light-emitting device using a gallium nitride based compound semiconductor comprises a gallium nitride based compound semiconductor layer grown on a substrate made of a material such as sapphire through crystal growth, an n-type gallium nitride based compound semiconductor, an active layer, and a p-type gallium nitride based compound semiconductor in sequence. In the light-emitting device having such structure, electrons in the n-type gallium nitride based compound semiconductor recombine with holes in the p-type gallium nitride based compound semiconductor in the active layer and lights generated in the active layer are outputted from the device.
As a light-emitting semiconductor device having such structure, a device having structures such as outputting lights directed to the upside of the active layer and outputting lights directed to the downside of the active layer through a transparent substrate has been known. Especially structure in which lights are outputted through a transparent substrate (hereinafter referred to as “flip-chip type”) uses an electrode layer formed on the p-type gallium nitride compound semiconductor as a reflection layer.
Generally, in a light-emitting device using a Group III nitride based compound semiconductor, electrodes for the n-type gallium nitride based compound semiconductor layer and the p-type gallium nitride based compound semiconductor layer, respectively, are formed on same side of a plane because such light-emitting device comprises an insulation substrate. There are two types of flip-chip type structure and wire-bonding type structure in which lights are outputted from the semiconductor layer side through the transparent electrode, in such a light-emitting device. Compared with a wire-bonding type light-emitting device, absorption of light in a light-transmitting electrode and screening of light by a thick-film pad electrode which is usually used in a wire-bonding device and is formed on the light-transmitting electrode hardly occur in a flip-chip type light-emitting device. That enables a flip-chip type device to output light generated in the active layer efficiently.
Conventinally, as shown in Japanese Patent Application Laid-open No. 2000-36619, a flip-chip type light-emitting semiconductor device using double-hetero junction structure of a Group III nitride based compound semiconductor is known. The conventional light-emitting device comprises a sapphire substrate as a light-transmitting substrate and an electrode layer made of rhodium (Rh) as a reflection layer and an electric current supplying layer formed on the uppermost p-type contact layer. Some of lights which is emitted from the emission layer directs to the opposite side of the substrate and is reflected by the electrode layer toward the substrate side to thereby outputting lights effectively. Rhodium has comparatively high reflectivity of 60% toward blue-color light. On the contrary, silver, aluminum, or a silver alloy has reflectivity of more than 60%, and they are useful for forming the reflection layer. However, silver and aluminum tarnish and deteriorate by carrying out heat treatment to the p-type contact layer such as alloying, which results in decreasing reflectivity. Especially, when silver is used to form the reflection layer, the Ag reflection layer may easily be exfoliated from the p-type contact layer. And because ion migration and electro migration in silver are large, problems may be left both in electricity and reliability.
As shown in Japanese Patent Application Laid-open No. 10-4208, a GaAl semiconductor also has similar problems. In order to solve such problems, a transparent indium tin oxide (ITO) is formed on the contact layer and Au or Au/Cu reflection layer is deposited thereon so that the metal can be kept from reacting directly with the semiconductor layer.
Japanese Utility Model Registered No. 3068914 discloses to form an ITO film on a GaN based semiconductor layer and then form a reflection layer made of silver or aluminum in order to improve external quantum efficiency of a flip-chip type light-emitting device.
Japanese Patent Application Laid-open No. H6-232450 discloses to deposit an electrode made of gold (Au)/nickel (Ni) having a width of 20 μm or less on a gallium nitride based compound semiconductor doped with p-type dopant, to carry out annealing treatment, and to deposit a conductive material such as Au, indium (In), and aluminum (Al) thereon.
Japanese Patent Application Laid-open No. 2001-217461 discloses a flip-chip type gallium nitride compound semiconductor light-emitting device comprising an electrode, in which a contact layer made of a metal material which can ohmic contact to the gallium nitride based compound semiconductor layer, a reflection layer made of Ag having high reflectivity, and a protection film made of Ni are deposited in sequence, formed on the p-type gallium nitride based compound semiconductor layer. Here, Ni, Co, and antimony (Sb) are disclosed as a metanl material having good ohmic contact characteristic.
Japanese Patent Application Laid-open No. 2003-224297 discloses a flip-chip type gallium nitride based compound semiconductor light-emitting device in which a light-transmitting ohmic electrode is formed on a p-type gallium nitride based compound semiconductor layer and a reflection layer covers the light-transmitting ohmic electrode through a light-transmitting insulation film. Here, Ni—Au, Ni—Pt, Co—Au are disclosed as materials to form the light-transmitting ohmic electrode, SiO2, Al2O3, SiN, TiO2, ZrO2 are disclosed as materials to form the light-transmitting insulation film, and Al, Ag and Rh are disclosed as materials to form the reflection layer.
Japanese Patent Application Laid-open No. 2004-71655 discloses a flip-chip type gallium nitride compound semiconductor light-emitting device in which an electrode is formed on the p-type gallium nitride based compound semiconductor layer. The electrode comprises a first member which has an opening part at the center and is made of a material selected from Ni, Au, Rh, Ir and Pd having low contact resistivity, and a second member which is formed on the first electrode member and is mainly made of Ag which fills the opening part of the electrode member.